“Did we hear a squeak? Was that a squeak?” she said in an interview, describing the back and forth.

With two other zoo officials, Dr Smith was counting the sounds — all of them squeals “like nothing you’ve ever heard before” – before she and her colleagues were ready to make an announcement. “Once you have a baby panda, the whole world gets involved,” she said.

In a video made on Sunday night, the mother, Mei Xiang, can be seen rustling in her dense nest of bamboo at 10:45 p.m. as she undergoes contractions.

“She gives birth,” said Don Moore, associate director of animal care sciences at the Smithsonian. “She is startled by something on the floor – that would be the cub — and she reaches down and takes hold of it.”
Mei Xiang had last borne a cub in 2005, and the seven-year gap had given zookeepers little reason to hope that she would do so again. Her age, 14, and a series of pseudopregnancies that never resulted in offspring had led experts to estimate that she had less than a 10 percent chance of giving birth.

So until the new baby emerged — “about the size of a stick of butter,” Dr. Moore said — officials were wary of trusting in the evident signs of pregnancy. Their uncertainty underlines the complex and somewhat uncharted nature of the captive breeding of panda bears.

First, there’s the delicate matter of courtship and mating. Having to rely on Mei Xiang’s mate, Tian Tian — “not the most confident breeder as a male,” Dr. Moore said — has made impregnation challenging.

In the wild, female pandas go into heat and mate with several males. When they go into heat in captivity, only one candidate is often available, usually one that lives apart from the female. That means that zookeepers must watch to determine when the female is most receptive to the male’s advances.

But in the event that the singular male on site is “shy,” as with Tian Tian, zoo officials may turn to artificial insemination. Pierre Comizzoli, a gamete biologist at the Smithsonian, was part of the insemination team that impregnated Mei Tiang with a batch of Tian Tian’s sperm that was frozen in 2005.

The team has a quality checklist. There must be a high density of sperm cells in the sample. The sperm need to move in a straight line. (Those that move in confused circles are less reliable.)

Anesthesia follows, and during her slumber, the female panda is inseminated. There’s an art to it all, but “the biggest challenge is that we still need to better understand the basic physiology of these animals,” Dr. Comozzoli said.

While the panda has long been part of the public consciousness — emblazoned, for example, on the W.W.F.’s crest — the creatures can still befuddle experts.

A female panda can exhibit all the signs of a successful pregnancy – raised hormone levels in her urine, weight gain and nesting behavior – and not give birth at the end of it all. Since she is in heat only once a year, these pseudopregnancies work like an “insurance policy for the real thing,” in case fertilization and implantation have actually occurred, researchers say.

But even when all goes well, the mother-to-fetus size ratio is so enormous that scientists must struggle to find the cub during ultrasound tests. (The Washington Post posted a helpful graphic on Monday explaining how the panda grows.)

So Mei Xiang’s cub was not altogether expected. “We are still giddy,” Dr. Smith said. “There’s still a little amazement and disbelief, a whole lot of elation. And then there’s the fact that we’re going to be watching this little cub growing up.”

It will be some time before zookeepers catch a glimpse. Mother pandas shield their young for weeks before they leave briefly to find food and water. But when Mei Xiang finally does go foraging, zookeepers will conduct the quickest possible physical exam — even if they have to take a second timer with them, Dr. Smith joked.10:38 a.m. | Updated The zoo reports that its staff did catch a few glimpses of the cub Monday night.

Some commentators fault the publicity surrounding the birth as a “cuteness overload” intended to encourage “Pandamania” and draw zoo donations. But Dr. Comozzoli counters that panda births inspire hope and open a window onto the natural world. And then there are the research benefits.

“Our female didn’t reproduce for several years,” he said. “So scientifically,” the birth ” is a huge source of information.”

Conservation-wise, the new cub joins a 350-strong pool of genetically diverse captive pandas, most of which are in China. The hope is that many could eventually be returned to the wild in China through programs that expose them to their natural bamboo forest habitat.

But for now, Dr. Moore said he simply rejoices in a small step forward. “It’s good news, when the world has a lot of bad news,” he said.

Here’s a video of the birth released by the zoo:

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